Emulsion polymerization is a heterogeneous process often used by industry to polymerize a wide variety of monomers using free radical mechanisms. It involves the polymerization of monomers in the form of emulsions or latexes. Polymers commonly formed by emulsion include acrylics, styrenics, polyvinylchloride (PVC), styrene-butadiene rubber, ethylene-propylenediene terpolymer-based (EPDM), polystyrene, acrylonitrile-butadiene-styrene copolymer (ASS), neoprene rubber, ethyl-vinyl acetate, styrene-maleic anhydride, tetrafluroethylene, and vinyl fluoride.
Generally, low molar mass ionic surfactants have enjoyed the most use in water-based emulsion polymerizations because they work so efficiently to stabilize the ionic double layer of the emulsion or colloid particles which prevents particle coagulation. In addition, polymeric surfactants have also been utilized to stabilize emulsion polymerizations. See Piirma, Polymeric Surfactants in 42 Surfactant Science Series (Marcel Dekker, New York 1992). This class of surfactants stabilizes colloidal particles by steric, rather than ionic, means. Steric stabilization of emulsions can be advantageous in threat (a) steric systems are much less sensitive to fluctuations and increases in electrolyte concentrations, (b) they work well at high and low solids contents, and (c) they stabilize aqueous and nonaqueous dispersions equally well. See Napper, Polymeric Stabilization of Colloidal Dispersions (Academic Press, New York 1983). Many nonionic polymeric surfactants are available; the most common of these are basically block copolymers of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO).
After polymerization, the polymer must be coagulated and isolated from the aqueous phase for further processing (except for that which is destined for use in water-borne coatings). The large volume of water remaining comprising the continuous phase must be properly handled, as it becomes contaminated with organic compounds-residual monomers, stabilizers, and other materials that are difficult to remove. As a result, it would be desirable to provide a different medium for the continuous phase that can be easily decontaminated.
In view of the foregoing, it is a first object of the present invention to provide a heterogeneous polymerization method in which a fluid other than water comprises the continuous phase medium.
It is also an object of the present invention to provide surfactants useful for the foregoing methods.
It is a further object of the present invention to provide initiators suitable for use with the foregoing methods.